Transcript Chapter 20 - William Stallings, Data and Computer

Data and Computer
Communications
Chapter 20 – Internetwork Quality
of Service
Ninth Edition
by William Stallings
Data and Computer Communications, Ninth
Edition by William Stallings, (c) Pearson
Education - Prentice Hall, 2011
Transport Protocols
At St. Paul's a great throng crammed the platform.
She saw a sea of faces, each stamped with a kind
of purposeful, hungry urgency, a determination to
get into this train. As before, when she was on the
Northern Line, she thought there must be some
rule, some operating law, that would stop more
than a limited, controlled number getting in.
Authority would appear and stop it.
—King Solomon's Carpet,
Barbara Vine (Ruth Rendell)
Transport Protocols
 Internet
traffic continues to grow and
change



demand for real-time responses
increasing use of audio, image, and video
heavy use of the World Wide Web
 packet-switching
technology with routers
functioning as switches was not designed
to handle voice and video
 strong need to support a variety of traffic
with QoS requirements within TCP/IP
Integrated Service
Architecture (ISA)
 intended
to provide QoS transport over IPbased Internets
 defined in RFC 1633
 portions already being implemented in
routers and end-system software
Internet Traffic - Elastic

traffic that can adjust, over wide ranges, to
changes in delay and throughput and still meet
the needs of its applications
 traditional type of traffic supported on TCP/IPbased Internets
 applications classified as elastic include:
Internet Traffic - Inelastic
 does
not easily adapt, if at all, to changes in
delay and throughput across an internet

real-time traffic
requirements for inelastic traffic include:
•
•
•
•
 new


Throughput
Delay
Jitter
Packet loss
internet architecture requirements:
resource reservation protocol
elastic traffic still needs to be supported
ISA

purpose is to enable
QoS support over IPbased internets
 sharing capacity
during congestion is
the central design
issue
 to
manage
congestion and
provide QoS
transport ISA
makes use of:




Admission control
Routing algorithm
Queuing discipline
Discard policy
ISA Components
ISA Services
 traffic
specification (TSpec)
three
categories
of service:
token
bucket
traffic
scheme
• Guaranteed
• Controlled load
• Best effort
• traffic sources can be defined
easily and accurately
• provides a concise description of
the load
• provides the input parameters to a
policing function
Token Bucket Scheme
Guaranteed Service
 key



elements are:
service provides assured capacity
specified upper bound on the queuing delay
through the network
there are no queuing losses
 application
provides a characterization
of expected traffic profile and the service
determines the end-to-end delay that it
can guarantee
 most demanding service provided by
ISA
Controlled Load
 key



elements are:
tightly approximates the behavior visible to
applications receiving best-effort service
under unloaded conditions
no specified upper bound on the queuing
delay through the network
high percentage of transmitted packets will be
successfully delivered
 useful
for adaptive real-time applications
Queuing Discipline
 routers
use first-in-first-out (FIFO) queuing
discipline

drawbacks of FIFO



no special treatment given to higher priority packets
smaller packets get delayed behind larger packets
a greedy TCP connection can crowd out more
altruistic connections
FIFO and
Fair
Queuing
Resource ReSerVation
Protocol (RSVP)
 RFC
2205
 provides supporting functionality for ISA
 prevention strategy


have unicast applications reserve resources in
order to meet a given QoS
enables routers to decide ahead of time if they
can meet the delivery requirement for a multicast
transmission
 must
interact with a dynamic routing
strategy

soft state
RSVP Characteristics
Receiver-Initiated Reservation

since receivers specify the desired QoS it makes
sense for them to make resource reservations



different members of the same multicast group may
have different resource requirements
QoS requirements may differ depending on the
output equipment, processing power, and link
speed of the receiver
routers can aggregate multicast resource
reservations to take advantage of shared path
segments
Soft State
 connectionless
 reservation
state is cached information in
the routers that is installed and periodically
refreshed
 if a new route becomes preferred the end
systems provide the reservation to the
new routers on the route
Data Flows
 Basis
of RSVP operation:
Session
• Destination IP address
• IP protocol identifier
• Destination port
Flow specification
• Service class
• Rspec
• Tspec
Filter specification
• Source address
• UDP/TCP source port
Data Flow Relationship
RSVP Operation
Reservation Attributes
and Styles
Reservation
Styles
RSVP Protocol
 RSVP
uses two basic message types:
Resv
originate at multicast
group receivers and
propagate upstream
create soft states within
routers that define
resources reserved for the
session
hosts set up traffic control
parameters for the first hop
Path
used to provide
upstream routing
information
reverse routing
each router and host creates a path
state that indicates the reverse hop
for source
RSVP Host Model
Differentiated Services (DS)
 RFC
2475
 designed to provide a tool to support a range
of network services
 key characteristics:


no change to IP is required
SLA is established prior to use of DS
• applications do not need to be modified

provides a built-in aggregation mechanism
• good scaling to larger networks and traffic loads


DS is implemented in individual routers
most widely accepted QoS in enterprise networks
Terminology
for
Differentiated
Services
DS Services
 typically
DS domain is under the control of
one administrative entity
 services provided across a DS domain are
defined in an SLA
Performance Parameters
in an SLA

detailed service performance parameters such
as expected throughput, drop probability, latency
 constraints on the ingress and egress points at
which the service is provided, indicating the
scope of the service
 traffic profiles that must be adhered to for the
requested service to be provided, such as token
bucket parameters
 disposition of traffic submitted in excess of the
specified profile
Services Provided in a SLA

traffic offered at service level A will be delivered with low
latency
 traffic offered at service level B will be delivered with low
loss
 ninety percent of in-profile traffic delivered at service
level D will be delivered
 traffic offered at service level E will be allotted twice the
bandwidth of traffic delivered at service level F
 traffic with drop precedence X has a higher probability of
delivery than traffic with drop precedence Y
DS Field
DS Domains
DS Traffic Conditioner
Expedited Forwarding PHB
(EF PHB)

RFC 3246
 building block for low-loss, low-delay, and low-jitter endto-end services through DS domains
 difficult to achieve
 cause is queuing behavior at each node
 intent is to provide a PHB in which packets encounter
short or empty queues
 configures nodes so traffic has a well-defined
minimum departure rate
Assured Forwarding PHB
(AF) PHB
 RFC
2597
 designed to provide a service superior to
best-effort but one that does not require
the reservation of resources within an
internet
 referred as to as explicit allocation

expands by defining four AF classes and
marking packets with one of three drop
precedence values
Service Level Agreements
(SLA)
 contract
between a network provider and a
customer that defines specific aspects of
the service that is to be provided
SLA includes:
• a description of the nature of service to be
provided
• the expected performance level of the service
• the process for monitoring and reporting the
service level
Framework for SLA
IP Performance Metrics
 IPPM
chartered by IETF to develop
standard metrics that relate to the quality,
performance, and reliability of Internet
data delivery
 need for standardization:


Internet has grown and continues to grow at a
dramatic rate
Internet serves a large and growing number of
commercial and personal users across an
expanding spectrum of applications
IP
Performance
Metrics
Packet Delay Variation
Summary
 Integrated


Elastic and inelastic traffic
ISA approach, components, services
 Resource


services architecture
reservation protocol
RSVP goals and characteristics
RSVP operation, filtering, reservation styles
 Differentiated

services
DS fields, configuration, operation
 Service
level agreements
 IP performance metrics